Climate classification - Atmosphere, Weather and Climate - page 416

Aridity criteria evaporation to temperature and precipitation. The

second element of the classification is an index of
thermal efficiency , expressed by the positive departure
of monthly mean temperatures from freezing-point.
Distribution for these climatic provinces in North
America and over the world have been published, but
the classification is now largely of historical interest.

Precipitation Steppe (BS)/ Forest/steppe

desert (BW) boundary

boundary

Winter precipitation r/t = 1 r/t = 2

maximum
Precipitation evenly r/ ( t 7) = 1 r/ ( t 7) = 2

B ENERGY AND MOISTURE BUDGET
CLASSIFICATIONS

distributed
Summer precipitation r/ ( t 14) = 1 r /( t 14) = 2
maximum

Thornthwaite's most important contribution was his
1948 classification, based on the concept of potential
evapotranspiration and the moisture budget (see
Chapters 4C and 10B.3c). Potential evapotranspiration
( PE ) is calculated from the mean monthly temperature
(in (C), with corrections for day length. For a thirty-day
month (twelve-hour days):

where: r = annual precipitation (cm)
T = mean annual temperature (°C)

The criteria imply that, with winter precipitation,
arid (desert) conditions occur where r/T < 1, semi-arid
conditions where 1 < r/T < 2. If the rain falls in summer,
a larger amount is required to offset evaporation and
maintain an equivalent effective precipitation.
Subdivisions of each major category are made with
reference, first, to the seasonal distribution of precipi-
tation. The most common of these are: f = no dry season;
m = monsoonal, with a short dry season and heavy rains
during the rest of the year; s = summer dry season;
w = winter dry season). Second, there are further tem-
perature criteria based on seasonality. Twenty-seven
subtypes are recognized, of which twenty-three occur in
Asia. The ten major Köppen types each have distinct
annual energy budget regimes, as illustrated in Figure
A1.1.
Figure A1.2A illustrates the distribution of the major
Köppen climate types on a hypothetical continent of
low and uniform elevation. Experiments using GCMs
with and without orography show that, in fact, the
poleward orientation of BS/BW climatic zones inland
from the west coast is determined largely by the western
Cordilleras. It would not be found on a low, uniform-
elevation continent.
The Köppen climatic classification has proved useful
in evaluating the accuracy of GCMs in simulating
present climatic patterns and as a convenient index
of change for climate scenarios projected for CO 2
doubling.
C. W. Thornthwaite introduced a further empirical
classification in 1931. An expression for precipitation
efficiency was obtained by relating measurements of pan

PE (in cm) = 1.6(10 t/I ) a
where: I = the sum for 12 months of ( t /5) 1.514
a = a further complex function of I .

Tables have been prepared for the easy computation of
these factors.
The monthly water surplus ( S ) or deficit ( D ) is deter-
mined from a moisture budget assessment, taking into
account stored soil moisture (Thornthwaite and Mather
1955; Mather 1985). A moisture index ( Im ) is given by:

Im = 100( S - D )/ PE

This allows for variable soil moisture storage according
to vegetation cover and soil type, and permits the
evaporation rate to vary with the actual soil moisture
content. The average water balance is calculated
through a bookkeeping procedure. The mean values
of the following variables are determined for each
month: PE , potential evapotranspiration, precipitation
minus PE ; and Ws , soil water storage (a value assumed
appropriate for that soil type at field capacity). Ws is
reduced as the soil dries (D Ws ). AE is actual evapotran-
spiration. There are two cases: AE = PE , when Ws is at
field capacity, or ( P - PE ) >0; otherwise AE = P + D Ws .
The monthly moisture deficit, D , or surplus, S , is deter-
mined from D = ( PE - AE ), or S = ( P - PE ) >0, when
Ws ≤ field capacity. Monthly deficits or surpluses are
carried forward to the subsequent month.